The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950° C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
U.S. Pat. No. 4,374,050 (Ray) discloses inert anodes made of specific multiple metal compounds which are produced by mixing powders of the metals or their compounds in given ratios followed by pressing and sintering, or alternatively by plasma spraying the powders onto an anode substrate. The possibility of obtaining the specific metal compounds from an alloy containing the metals is mentioned.
U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes non-carbon anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of a cerium compound to the molten cryolite electrolyte. This made it possible to have a protection of the surface from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer. Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve.
U.S. Pat. No. 5,510,008 (Sekhar/Liu/Duruz) discloses an anode made from an inhomogeneous porous metallic body obtained by micropyretically reacting a metal powder mixture of nickel, iron, aluminium and optionally copper. The porous metal is anodically polarised in-situ to form a dense iron-rich oxide outer portion whose surface is electrochemically active. Bath materials such as cryolite which may penetrate the porous metallic body during formation of the oxide layer become sealed off from the electrolyte and from the active outer surface of the anode where electrolysis takes place, and remain inert inside the electrochemically-inactive inner metallic part of the anode.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry for commercial aluminium production because their lifetime must still be increased.